The basic operation and structure of cellular and trunked communication are known. In trunked communication systems, a central resource controller allocates a limited number of channels (i.e., resources) among many communication units (e.g., subscribers in a trunked communication system) based on channel availability. For example, radios used as communication units in a trunked system are operative on a number of frequencies, which are assigned to each communication unit based on availability. Trunked communications systems typically comprise a communication resource controller, a plurality of repeaters, and a plurality of communication units which may be mobile vehicle radios or portable radios. Typical trunked communication systems have a relatively large geographic coverage.
A cellular communication system typically comprises a mobile telephone switching office, a limited number of communications channels, and a plurality of communications units that may be cellular telephones. Each of the plurality of cells comprises some of a limited number of communication channels, wherein one of the communication channels is designed for that cell. The control channel transceives operational information between the plurality of communication units within the cell and the mobile telephone switching office such that the communication units can place calls via a telephone system. The coverage area of each typical cell is relatively small in comparison with that of a typical trunked system.
Because of the different configurations of trunked and cellular communications systems, a typical communication unit, without two receivers and two transmitters cannot operate on both systems. Having two receivers and two transmitters adds expense to the manufacture of the unit, and accordingly a more expensive communication unit results. Presently, the United States Federal Communications Commission has designated the frequency range of 806 to 825 MHz as transmit frequencies and 851 to 870 MHz as receive frequencies for trunked communications systems, and 825 to 845 MHz as the transmit frequencies, and 870 to 890 MHz as receive frequencies for cellular communications systems. However, known communication units with one receiver and one transmitter are limited to operating in either one system or the other. Therefore, a need exists for a communication unit having one receiver and one transmitter that can alternately operate in either a cellular or a trunked communication system.
If one were to combine cellular and trunked modes in a communication device having a single receiver and a single transmitter, problems would arise due to the wide frequency range of operation. One such problem is the frequency synthesizer voltage-controlled oscillator (VCO) linearity. As the range of the control voltage of the VCO increases the synthesizer may begin to operate outside of its linear region. This is a problem because operation within the linear region is desirable for good performance. An additional problem arises in relation to the audio processing required for operation in the cellular mode. Cellular operation requires that the audio be expanded and compressed to overcome the problem of fading encountered at cellular radio frequencies. Another problem arises from the different protocols used in cellular and trunked communication systems.